Class 11 Chemistry - CBSE
Classification of Elements and Periodicity in Properties
The chapter Classification of Elements and Periodicity in Properties forms the backbone of inorganic chemistry for Class 11 CBSE students. It explores the historical attempts to organize elements, leading to the modern periodic law and the long form of the periodic table based on atomic numbers. You will study electronic configurations of elements in periods and groups, and analyze periodic trends in physical and chemical properties such as atomic radius, ionization enthalpy, electron gain enthalpy, electronegativity, and valence. Mastering these concepts is crucial for board exams as it explains chemical reactivity and forms the basis for bonding in later chapters.
Start Learning FreeKey Concepts
Modern Periodic Law
The physical and chemical properties of elements are periodic functions of their atomic numbers, resolving anomalies found in Mendeleev's table which was based on atomic weights.
Atomic and Ionic Radii
Atomic radius generally decreases across a period due to increasing effective nuclear charge and increases down a group due to the addition of principal energy shells.
Ionization Enthalpy
The energy required to remove an electron from an isolated gaseous atom in its ground state; it increases across a period and decreases down a group.
Electron Gain Enthalpy
The enthalpy change when an electron is added to a gaseous neutral atom, becoming more negative across a period and less negative down a group.
Electronegativity
The qualitative measure of an atom's ability to attract shared electrons in a covalent bond, with Fluorine being the most electronegative element.
Important Formulas
Board Exam Info
In the CBSE Class 11 Chemistry board exams, this chapter typically carries around 4 to 6 marks. Common question types include reasoning questions based on periodic trends (e.g., comparing size of cations/anions, explaining successive ionization enthalpies), identifying element positions from electronic configurations, and predicting chemical formulas.
Frequently Asked Questions
Why is the radius of a cation smaller than its parent atom while an anion is larger?
A cation is smaller because the loss of electrons results in a higher effective nuclear charge pulling the remaining electrons closer. An anion is larger because added electrons increase electron-electron repulsion, causing the electron cloud to expand.
Why does second ionization enthalpy always have a higher value than first ionization enthalpy?
After the removal of the first electron, the remaining electrons are held more tightly by the nucleus due to a reduced number of electron-electron repulsions and a constant nuclear charge, requiring more energy to remove a second electron.
Why do noble gases have positive electron gain enthalpies?
Noble gases have stable, fully filled electronic configurations (octet or duplet), making it energetically unfavorable to add an extra electron, thus requiring energy addition.
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